Synthesis of ionic liquids

ABSTRACT

Ionic compounds which are liquids at room temperature are formed by the method of mixing a neutral organic liqand with the salt of a metal cation and its conjugate anion. The liquids are hydrophobic, conductive and stable and have uses as solvents and in electrochemical devices.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with United States Government support under Contract No. DE-AC05-960R 22725 between the United States Department of Energy and Oak Ridge National Laboratory, managed by UT-Battelle, LLC, and the United States Government has certain rights in this invention.

This invention relates to new synthetic routes to the preparation of hydrophobic ionic liquids and the ionic compounds made thereby.

BACKGROUND AND PRIOR ART

Ionic liquids are organic salts with melting points below 100° C. and typically are liquids at room temperature. Early interest in the compounds was based upon their conductivity, as described in U.S. Pat. No. 4,764,440. Ionic liquids may be used as a solvent in a chemical vapor deposition system (U.S. Published Patent Application No. 2002/0001674), as coupler solvents in photothermographic systems (U.S. Pat. No. 6,531,270), as solvents for Friedel-Crafts and Diels-Alder reactions (U.S. Pat. No. 6,573,405), as a catalyst for isomerisation reactions, (U.S. Published Patent Application No. 2003/0109767), as complexing agents in separations (U.S. Published Patent Application No. 2003/0125599 and U.S. Pat. No. 6,623,659) as a solvent to form regenerated cellulose (U.S. Published Patent Application No. 2003/0157351), and as a polymerization catalyst (WO 03/087390), to name a few.

Ionic liquids may be made by the reaction of an onium chloride with a Lewis acid such as AlCl₃. Heterocyclic halides react with lithium borates in acetonitrile to form ionic liquids useful in electrochemical cells (U.S. Published Patent Application No. 2002/0015883) and with lithium trifluorophosphates to form inert solvents (U.S. Published Patent Application No. 2002/0015884). EMICI (1-methyl-3-ethyl imidazolium chloride) may be reacted with potassium bis-fluorosulfonimide (KFSI) to yield a conductive liquid useful as a current collector (U.S. Pat. No. 6,365,301). Sulfonated or carboxylated triesters of phosphorous acid may serve as anions for ammonium cations (U.S. Published Patent Application 2002/0161261). Salts of diazonium, sulfonium, iodonium or metallocennium types may be useful in chiral syntheses (U.S. Pat. No. 6,548,567).

An aqueous nitrate of Ag(I) may be reacted with an imidazolium chloride to form an ionic liquid and a silver chloride salt (U.S. Pat. No. 6,379,634). A halide-free ionic liquid may be obtained by reacting a halide salt of an organic cation with a Br{acute over (ø)}nsted acid in an alcohol or hydrocarbon solvent (WO 03/051874).

A two-step continuous process is disclosed in WO 03/089389. WO 03/093246 describes liquids wherein the cation is a nitrogen or phosphorous compound and the anion is a five-member nitrogen heterocycle. A process to minimize halides in ionic liquids is based on fluorinated esters or alkyl sulfonates as replacements for haloalkanes when forming an imidazolium salt (U.S. Published Patent Application No. 2003/0080312) and lower melting temperatures have been obtained when the cation is Zn, Sn or Fe (III) and the anion is a quaternary amine (U.S. Pat. No. 6,573,405).

Chiral ionic liquids may be made from optically active ammonium cations and used for asymmetric syntheses (U.S. Published Patent Application No. 2003/0149264). Metallic cations and perhalogenated substituents on the anionic portion are disclosed in U.S. Pat. No. 6,620,546.

In consideration of the many uses for ionic liquids, a need exists for liquids with different properties with new uses and for new ways to make them.

BRIEF SUMMARY OF THE INVENTION

The invention relates to new methods for the synthesis of ionic compounds, especially liquids, and to the new liquids made by the methods. These liquids are salts that are liquid at room temperature, hence RTIL. The liquids are hydrophobic and compatible with extraction processes and reaction schemes in organic chemistry.

The objectives of this invention may be met using the complexation of cations by neutral liqands. This produces room temperature ionic liquids having cationic coordination metal complexes.

DETAILED DESCRIPTION OF THE INVENTION

Crown ethers are readily available commercially and used primarily in chemical research because the exposed oxygen atoms readily complex with metal ions. Depending on substituents, the crown ethers may have adjustable solubility in aqueous solvents.

When reacted with an alkaline organic salt, crown ethers form coordination metal complexes of the ether and the alkaline metal, together with an organic anion gegenion. The organic salt of many of these compounds is a room temperature ionic liquid with a low volatility and is strongly hydrophobic.

Suitable crown ethers for this purpose are shown in FIG. 1. Reference is made also to catalogues from Aldrich, Gelest and Tokyo Kasei, Kogyo.

Suitable metals are sodium, potassium, lithium and calcium

These reactions are exothermic and require no solvent, heat or catalyst. Excess reagent salt can be washed away.

A similar system may be formed using small cations with neutral organic ligands in what formally appear to be a methathesis reactions, an exchange of anions. Organic amines are representative of the neutral liqand. Silver is a representative small cation and forms stable complexes with amines. Salts such as lithium bis(trifluoromethane)sulfonimide are suitable for exchange because they supply a suitable bulky anion. Such systems are readily worked-up using water to remove salt residues.

Table 1 shows representative examples of alkyl amine salts, yields and properties.

Suitable metal ions include Ag⁺¹, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Hg²⁺, and Fe³⁺.

The structural features of the cations of these ionic liquids are given in FIG. 2.

Other neutral liqands for purposes of this invention include sulfur and phosphorous compounds containing neutral liqands.

EXPERIMENT 1

Neat cyclohexyl-15-crown-5 (Parish, Inc.), was mixed with an equimolar amount of N-lithiobis(trifluoromethane)sulfonimide Li(Tf)₂N in a boiling flask at room temperature without an inert blanket and stirred using a magnetic stir bar. Warming was apparent tactilly and a clear colorless solution obtained.

The same compounds under the same conditions were reacted at a ratio of cyclopexo-15 crown-5 to Li(Tf)₂N of 1:1.35.

No loss of mass was observed during vacuum rotary evaporation at 100° C. for four hours.

Both products were soluble in organic solvents including acetone and acetonitrile but immiscible in water and aqueous solutions.

FIG. 3 shows comparative FTIR spectra of neat cyclohexyl-15-crown-5 (a) of the room temperature ionic liquid obtained by the reaction of the crown ether with Li(Tf)₂N 1:1 (b) and by 1:1.35 reaction of the ether with Li(Tf)₂N. The peak in the region of 2900 cm⁻¹ of the neat ether, corresponding to a C—H stretch, has been shifted by complexation as shown in the figure. This is evidence of the complexation of the ether with the lithium cation.

FIG. 4 shows the comparative Raman spectra in the C—H stretching region of the pure cyclohexyl-15-crown-5 (a) and the RTIL of the 1:1 complex with Li(Tf)₂N (b).

EXAMPLE 2

Compounds according to Table 1 were obtained by mixing amines of the formula R₁, R₂—NH₂ with 1:1 aqueous solution of AgNO₃ in D.I. water at room temperature with stirring. A stoichometric amount, based upon amount of R—NH₂ of Li(Tf)₂N was added to a stirred solution of the Ag (H₂NR,) (H₂NR₂)obtained from the first step was added with stirring and the mixture was stirred for one hour and then poured into a separatory funnel. The lower layer of water containing dissolved LiNO₃ was drawn off. The RTIL obtained was washed three times with D.I. water and dried using a vacuum rotary evaporator at 80° C. for six hours. The dried product was weighed and the yield calculated based upon Ag.

Table lists the various R-groups used, the yield, density and conductivities measured using a conductivity meter.

FIG. 5 shows the Raman spectra of propylamine (a) and Ag(H₂N—C₃H₇)₂+Tf₂N (b).

FIG. 6 is the proton nmr spectrum of Ag(H₂N—C₃H₇)₂+(Tf)₂N in deuterated chloroform showing the shifts of the amino, ethyl and methyl propyl amine protons and the splitting patterns, together with peak integrations.

FIG. 7 is the nmr spectrum for Ag(NH₂CH₃)+(R₁═CH₂R₂═H;

FIG. 8 is the nmr spectrum for R₁═R₂═CH₃;

FIG. 9 is nmr spectrum for R═R₂=tert-butyl;

FIGS. 10 through 15 are the proton nmr spectra for mixed amines;

FIG. 10 is for R₁═CH₃, R₂═CH₃═CH₂;

FIG. 11 is for R₁═CH₃, R₂═CH₃CHR₂CH₂;

FIG. 12 is for R₁═CH₃CH₂CH₂R₂=tert-butyl;

FIG. 13 is for R₁═CH₃CH₂, R₂═CHR₃CH₂CH₂;

FIG. 14 is for R₁═CH₃CH₂CH₂, R₂=tert-butyl;

FIG. 15 is for R₁═CH₃CH₂, R₂=tert-butyl.

Draft

It is noted that in all cases the shifts, splits and integrations shown in the figures are consistent and predictable for the structures.

FIG. 15 is the carbon-13 nmr of R₁═R₂═CH₃CH₂CH₂ in deuterated chloroform. Whereas the proton shifts were determined at 400.13 MHz, these data were obtained at 100.61 MHz. The internal standard for both sets of spectra was tetramethysilane (TMS). The expected fluoride shifts for the trifluoromethyl group is quite discernable.

The RTIL of this invention are unique because they are the first such liquids having an inorganic cation complexed with a neutral organic liqand. They have conductivities comparable to the traditional EMI+ salts but are formed by different processes allowing a greater tuning by changing substituents on the organic liqand. These compounds are believed to be useful cosolvents in the separation of metal salts from contaminated aqueous systems, especially systems contaminated with soluble radioactive compounds such as those with strontium slats. They are also useful in the separation of alliances from olefins, with particular application to propane: propylene system. This invention has been described in terms of representative examples. Modifications and additions obvious to those with skills in the art are subsumed within the scope of the invention. TABLE 1 Densities, conductivities, and synthesis yields for (R₁—NH₂ →Ag⁺

H₂N—R₂)Tf₂N⁻ionic liquids. Synthesis Density Conductivity R₁ R₂ Yield at 24° C. mS/cm at 24° C. Methyl Methyl 68.2 1.86 12.30 Ethyl Ethyl 78.2 1.73 8.07 Propyl Propyl 93.0 1.63 3.98 t-Butyl t-Butyl 76.6 N/A^(a) N/A^(a) Methyl Ethyl 76.3 1.80 9.70 Methyl Propyl 82.6 1.74 6.42 Methyl t-Butyl 79.1 N/A^(a) N/A^(a) Ethyl Propyl 85.6 1.69 5.00 Ethyl t-Butyl 86.0 1.63 2.20 Propyl t-Butyl 85.5 1.55 1.70 ^(a)The corresponding salts are solid at room temperature.

A similar system may be formed using small cations with neutral organic ligands in what formally appear to be a methathesis reactions, an exchange of anions. Organic amines are representative of the neutral liqand. Silver is a representative small cation and forms stable complexes with amines. Salts such as lithium bis(trifluoromethane)sulfonimide [(CF₃SO₂)₂N—Li, “lithiotrifluorosulfonylamide, Li (Tf₂)N], BF₄ ⁻, NO₃ ⁻, SO₄ ⁻, PO₄ ⁺³, PF₆ ⁻ and dicyanamide [N(CN)₂ ⁻] are suitable for exchange because they supply a suitable bulky anion. Such systems are readily worked-up using water to remove salt residues.

Suitable metal ions include Ag⁺¹, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Hg²⁺, Co³⁺ ions and Fe³⁺.

The same compounds under the same conditions were reacted at a ratio of cyclohexyl-15-crown-5 to Li(Tf)₂N of 1:1.35.

The RTIL of this invention are unique because they are the first such liquids having an inorganic cation complexed with a neutral organic liqand. They have conductivities comparable to the traditional EMI+ salts but are formed by different processes allowing a greater tuning by changing substituents on the organic liqand. These compounds are believed to be useful co-solvents in the separation of metal salts from contaminated aqueous systems, especially systems contaminated with soluble radioactive compounds such as those with strontium₁, cesium, silver, copper and lanthanum salts. They are also useful in the separation of alkanes from olefins, with particular application to propane: propylene system. This may be useful as liquid separation membrane for gasses, as sensing transducers, electrolyte for super capacitors, as stationary phases for chromatography and as heat transfer fluids. This invention has been described in terms of representative examples. Modifications and additions obvious to those with skills in the art are subsumed within the scope of the invention. 

1. An ionic compound comprising a cation which is a complex of a neutral liqand selected from the group consisting of organic amines and crown ethers with a metal ion selected from the group consisting of Ag⁺, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Hg²⁺, Co³⁺ and Fe³⁺ and an anion which is a conjugate anion of the metal ion.
 2. An ionic compound according to claim 1 which is a liquid below 100° C.
 3. An ionic compound according to claim 2 which is a liquid at room temperature.
 4. An ionic compound according to claim 1 which is electrically conductive in the absence of a solvent.
 5. An ionic liquid according to claim 1 which is hydrophobic.
 6. An ionic compound according to claim 1 wherein said neutral organic liqand is a crown ether.
 7. An ionic liquid according to claim 1 wherein the neutral organic liqand is at least one alkyl amine.
 8. An ionic compound according to claim 1 wherein said conjugate anion is bis(trifluoromethane)sulfonimide, boron trifluoride, nitrate, sulfate, phosphate, hexafluorophosphate and dicyanamide.
 9. A method for forming an ionic liquid comprising mixing a neutral liqand selected from the group consisting of organic amines and crown ethers with a metal ion selected from the group consisting of Na⁺, K⁺, Li⁺, Ca²⁺, Ag⁺, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Hg²⁺, and Fe³⁺ and with the salt of a metal cation and its conjugate anion at room temperature.
 10. A method according to claim 9 wherein said neutral organic ligand is a crown ether.
 11. A method according to claim 10 wherein the metal cation is selected from the group consisting of sodium potassium, lithium and calcium.
 12. A method according to claim 9 wherein said neutral organic ligand is an alkylamine.
 13. A method according to claim 12 wherein said metal cation is selected from the group consisting of silver, zinc, copper, cadmium, nickel, mercury and iron.
 14. A method according to claim 9 wherein said conjugate anion is bis(trifluoromethane)sulfonamide, boron trifluoride, nitrate, sulfate, phosphate, hexafluorophosphate and dicyanamide, selected from the group consisting of organic amines and crown ethers with a metal ion selected from the group consisting of Na⁺, K⁺, Li⁺, Ca²⁺, Ag⁺, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Hg²⁺, and Fe³⁺ and an anion which is a conjugate anion of the metal ion.
 15. A method according to claim 9 which is performed at room temperature.
 16. An ionic compound according to claim 1 which may be used as a solvent.
 17. An ionic compound according to claim 1 which may be used for gas liquid separation.
 18. An ionic compound according to claim 1 which may be used for solvent extraction.
 19. An ionic compound according to claim 4 which is used in electrical devices.
 20. An ionic compound according to claim 1 which is used as a heat transfer fluid. 